This invention relates generally to satellite-based communications systems and, in particular, to satellite communications systems that employ a constellation of non-geosynchronous orbit (NGSO) satellites, a plurality of user terminals, and a plurality of gateways for interconnecting voice or data calls between individual ones of the user terminals and terrestrial communication networks via one or more satellites of the NGSO constellation.
The above-referenced U.S. Pat. No. 5,592,481 describes a satellite communication system that employs terrestrial gateways and a constellation of NGSO satellites, such as Low Earth Orbit (LEO) satellites, for linking user terminals to, for example, the Public Switched Telephone Network (PSTN). Each gateway has a gateway coverage area and services those user terminals that reside permanently or temporarily within the coverage area. A call that arrives at the gateway from the PSTN for a given user terminal is connected to the user terminal via one or more satellites. In like manner a call originated by the user terminal is connected to the PSTN through the gateway and one or more satellites. This system employs a Code Division Multiple Access (CDMA) modulation scheme and enables the gateway to set up multiple links to and from the user terminal through two or more satellites using two or more directional gateway antennas. This provides satellite diversity, as the user terminal (and gateway) are enabled to coherently combine two or more received signals, thus mitigating the effects of signal blockage and fading.
Referring to FIG. 1 of this patent application, the above-referenced U.S. Patent generally teaches a satellite communications system wherein a gateway (GW) 1 bidirectionally communicates with a user terminal (UT) 3 via NGSO satellites (e.g., at least SAT 1 and SAT 2). SAT 1 and SAT 2 may be bent pipe repeaters, or may use on-board processing and may contain regenerative repeaters. The gateway 1 is bidirectionally coupled to a terrestrial communications system, such as the Public Switched Telephone Network (PSTN), as well as to a Ground Data Network (GDN) which provides an ability to communicate with a Ground Operations Control Center (GOCC), which is not shown. The gateway 1 includes a plurality of directional antennas, e.g., antennas 1A and 1B, while the user terminal 3 includes, typically, an omni-directional antenna 3A. For the case of a voice or data call that is connected to the PSTN the gateway 1 can provide multiple satellite diversity by routing Code Division Multiple Access (CDMA) forward links through at least SAT 1 and SAT 2.
As is also disclosed in copending U.S. patent application Ser. No. 08/903,166, filed Jul. 3, 1997 now U.S. Pat. No. 5,796,760 dated Aug. 18, 1998, entitled xe2x80x9cMultipath Communication System Optimizerxe2x80x9d, by Robert A. Wiedeman and Paul A. Monte, which is a continuation of U.S. patent application Ser. No. 08/239,750, filed May 9, 1994 now U.S. Pat. No. 5,859,874 dated Jan. 12, 1999, the disclosures of which are incorporated by reference herein in their entireties, each forward link (FL) has a unique spreading code that enables the link to be separately despread, demodulated and combined in the user terminal 3. This provides multiple satellite diversity for the user terminal 3, and facilitates overcoming signal blockage and fading conditions that may occur between the user terminal 3 and one or more of the satellites. The user terminal 3 may thus include a multi-finger decorrelator such as a Rake receiver. Two or more fingers can be used for receiving forward traffic channels from two or more of the satellites, while another finger can be time multiplexed to receive pilot signals transmitted by the gateway 1 through the different satellites. If available, a plurality of fingers can be used for this purpose.
In the system disclosed in U.S. Pat. No. 5,592,481 a technique is provided for allocating communications traffic through the plurality of satellites of the NGSO constellation. Each of the plurality of satellites is oriented, at any given time when in view of the user terminal, at a particular elevation angle. A method includes steps of: (a) providing each of the plurality of satellites with a receiver for receiving communication links from the ground station and a transmitter for transmitting communication links to user terminals; (b) in response to a request for service, determining if a highest elevation angle satellite can be assigned a new communications link; (c) if yes, assigning a new communication link to the highest elevation angle satellite; (d) if no, determining if a second highest elevation angle satellite can be assigned a new communications link; and (e) if yes, assigning a new communication link to the second highest elevation angle satellite. A number of different criteria can be employed in determining if a satellite can be assigned a new communication link, including: determining if the associated satellite has already been assigned a predetermined maximum number of communication links; and determining if the associated satellite, or a particular beam, is transmitting at or near to a power level that corresponds to a maximum peak flux density at the surface of the earth. Each of the steps of assigning is preferably accomplished such that the communication link is simultaneously established through at least two of the satellites to provide for diversity reception at a user terminal. This method is extended to a case of assigning a new satellite during the handoff of the user terminal to a another satellite, as well as to a case of assigning additional satellites to enhance the satellite diversity provided to the user terminal.
It would be desirable to extend these teachings so as to provide additional criteria that can be considered during the assignment of one or more satellites to the user terminal.
It would also be desirable to extend these teachings such that a user terminal would be enabled to transition from a first gateway coverage area to a second gateway coverage area during a call, without dropping the call or requiring that the call be somehow switched from the first gateway to the second gateway, as well as to optimally assign communication satellites to the user terminal. For example, it is envisioned that during a call some mobile user terminals, such as those contained within automobiles, trains and aircraft, will cross political and other boundaries that also happen to define boundaries between adjacent gateway coverage areas. Alternatively, and for the case of a user that is relatively stationary, the user may be better served by either of two gateways, depending on co-visible satellites.
It is a first object and advantage of this invention to provide a satellite communication system that enables a user terminal to simultaneously use and be assigned to satellites that are in view of the user terminal and at least two gateways, at the same time or not at the same time.
It is another object and advantage of this invention to extend the teachings of the above-referenced U.S. Pat. No. 5,592,481 to provide additional criteria upon which to base an assignment of a new communication satellite or satellites to a user terminal.
The foregoing and other problems are overcome and the objects are realized by methods and apparatus in accordance with embodiments of this invention.
In one aspect this invention provides a method for operating a satellite communications system, and a system that operates in accordance with the method.
The method includes steps of (a) establishing a call connection between a terrestrial telecommunications network and a user terminal via a first gateway and at least one satellite, (b) while the call connection is established, selecting at least one further satellite, and (c) maintaining the call connection to the terrestrial telecommunications network via a second gateway, the least one further satellite, and the first gateway. The at least one further satellite is selected, at least in part, based on information transmitted from the second gateway to the first gateway through an inter-gateway communications link.
The at least one further satellite need not be a new satellite, but could be a satellite that is going out of visibility at the first gateway and that is visible at the second gateway, and that will remain visible at the second gateway for at least some period of time.
In one embodiment the step of selecting includes steps of generating signal quality indications in the user terminal; transmitting the signal quality indications back to the first gateway directly and via the second gateway; and selecting the at least one further satellite at the first gateway based at least in part on the signal quality indications.
In another embodiment the step of selecting includes steps of generating signal quality indications at the first gateway based on signals received from the user terminal at the first and second gateways; and selecting the at least one further satellite at the first gateway based at least in part on the signal quality indications.
In a further embodiment the step of selecting includes steps of generating signal quality indications at the first gateway based on signals received from the user terminal at the first gateway; generating signal quality indications at the second gateway based on signals received from the user terminal at the second gateway; transmitting the generated signal quality indications back to the first gateway from second gateway; and selecting the at least one further satellite at the first gateway based at least in part on the signal quality indications.
In another embodiment the step of selecting includes steps of calculating at least one of current or future path gains at the first gateway based on signals received from the user terminal at the first and second gateways; and selecting the at least one further satellite at the first gateway based at least in part on the calculated path gains.
In a further embodiment the step of selecting includes steps calculating at least one of current or future path gains at the first gateway based on signals received from the user terminal at the first gateway; calculating at least one of current or future path gains at the second gateway based on signals received from the user terminal at the second gateway; transmitting the calculated path gains back to the first gateway from second gateway; and selecting the at least one further satellite at the first gateway based at least in part on the calculated path gains.
In all of these embodiments the step of selecting may further consider at least one of satellite elevation angle relative to the user terminal, satellite loading, satellite power consumption, and satellite transmitted power flux density.
The foregoing multi-gateway embodiments are also applicable to a single gateway case, wherein the inter-gateway communication link, or some other link, is not used for conveying the information between the gateways.